Tory MPs have been ordered to block a move that would reveal exactly who was behind a £425,000 donation made to the DUP during the Brexit referendum.

They will be whipped to vote against a motion when it comes before Parliament on Wednesday.

If passed, it would force Northern Irish political parties to publish full details of their donations for the first time - back to 2014.

But Tory MPs have been instructed to vote so the rule is only backdated to 1 July 2017.

As its confidence and supply partner, the DUP is expected to vote with the Government and achieve a majority.

The elections watchdog has recommended the new law is backdated to 2014, saying in a briefing note to MPs released last month that "transparency is an essential component of increasing public confidence in the democratic process".

It added: "Information on how political parties, candidates and other campaigners raise and spend money should be open to timely public scrutiny."

Owen Smith, Labour's shadow Northern Ireland secretary, said people deserved to know where funds - including the substantial donation to the DUP - came from.

"All parties in Northern Ireland apart from the DUP support the Government's previous promise to publish," he said.

"There is simply no excuse to not publish the donations.

"The Tories must explain why they are doing the DUP's dirty work by helping them avoid publishing the source of the funds received in the EU referendum.

"Those funds played a significant part in the referendum campaign across the UK and the public have a right to know precisely where that money came from."

A large portion of the £425,000 was spent outside Northern Ireland on a national advertising campaign backing Vote Leave, according to The Times.

The donation to the DUP - rather than the official Brexit campaign - means the source did not have to be revealed.

The party has volunteered that the donation came from the Constitutional Research Council, described as a "pro-Union, unionist organisation based in Great Britain".

If the law is successfully changed, the individual behind the donation would have to be named.

The Government tried to push the change through on Monday as a statutory instrument - a mechanism to pass provisions of an act of parliament without a debate in the Commons.

But after uproar from the Labour benches, a vote was tabled for Wednesday afternoon. It was due to follow Prime Minister's Questions.